1. Field of the Invention
The present invention relates to a structure of a push-button switch for use in electronic devices such as home electric appliances, business machines, and communication devices. Particularly, the invention is concerned with a structure of a push-button switch which is required to have a dust resistance.
2. Description of the Prior Art
A conventional structure of a push-button switch is shown in FIGS. 6 to 8, of which FIG. 6 is a sectional view of the push-button switch, FIG. 7 is a sectional view of the push-button switch as seen in a direction different from FIG. 6, and FIG. 8 is a perspective view of a fixed contact formed by a wire rod.
In these figures, the push-button switch is made up of a generally cylindrical, bottomed housing 11 formed by molding a synthetic resin, a pair of fixed contacts 12 exposed to an inside bottom of the housing 11, a fixed terminal 13 integral with the fixed contacts 12 and projecting to the exterior of the housing 11, a movable contact 14 formed by a dome-like inversion spring incorporated within the housing 11 and capable of moving into contact with and away from the fixed contacts 12, a stem 15 formed of a synthetic resin, the stem 15 being mounted on the movable contact 14 and capable of moving into the housing 11, and a cover formed by a metallic plate and fixed to an upper surface of the housing 11.
The fixed contact 12 and the fixed terminal 13 are integrally formed using the same wire rod. The wire rod is a single round wire rod having been subjected to a thickness deviation working. A total of two such round wire rods are used in a partially embedded state in the housing 11.
The two round wire rods, which extend in parallel with each other, are formed with two raised portions 12a, respectively, which are raised upward by a thickness deviation working. By inserting these round wire rods into a cavity of a mold and molding the housing 11, the raised portions 12 exposed to the inside bottom of the housing are used as the fixed contacts 12 and the round wire rods are used as the fixed terminals.
By this insert molding, the bottoms of the raised portions 12a are embedded in the housing 11, so that the round wire rods are difficult to turn with respect to the housing 11 and difficult to be disengaged from the housing.
In the push-button switch described above, when an operator depresses the stem 15, the movable contact 14, which is opposed to the inside bottom of the housing 11, undergoes a depressing force and is inverted into contact with both fixed contacts 12 (raised portions 12a), so that the fixed contacts 12 are rendered conductive with each other through the movable contact 14, thus causing the switch to turn ON.
With the switch ON, if the depressing force for the stem 15 is released, the movable contact 14 which has been inverted reverts to the original dome shape by virtue of its own elasticity. Consequently, both fixed contacts 12 cease to conduct and the switch turns OFF. At this time, the stem 15 undergoes a biasing force of the movable contact 14 and is pushed up to its initial position.
In the above conventional push-button switch structure, however, since the raised portion 12a of each round wire rod is constituted as the fixed contact 12, dust which has entered the switch may adhere to the fixed contact 12 (raised portion 12a) and cause a poor conduction, thus giving rise to a problem in point of contact reliability.